Tuberculosis is a major public health problem. The causative agent of tuberculosis, Mycobacterium tuberculosis (Mtb), currently infects nearly one third of the human population and causes over 2 million deaths annually. For reasons that are not yet fully understood, the immune system fails to completely clear the infection, which persists in macrophages. The broad objective of this proposal is to elucidate mechanisms regulating immunity during tuberculosis. Control of Mtb infection is attributed to T helper type 1 CD4 T cells which secrete interferon gamma (IFNg). IFNg activates macrophages inducing the production of reactive nitrogen and oxygen intermediates which, in turn, limit bacterial replication and dissemination. IFNg has regulatory effects on many other cell types as well. While CD4 T cells and IFNg are both abundant in the lungs during tuberculosis, we currently do not know if and how IFNg regulates CD4 T cell responses during this infection. Using a well-established mouse model of tuberculosis we will test the hypothesis that IFNg acts directly on CD4 T cells to regulate their differentiation, function and turnover during Mtb infection. We will compare Mtb-specific CD4 T cells that do or do not express a functional IFNg receptor. This approach will allow us to address the direct effects of IFNg on CD4 T cells in an otherwise normal host environment (ie: all other cell types express a functional IFNg receptor). In Aim 1 we will analyze expansion and differentiation of wild type and IFNg receptor-deficient Mtb-specific CD4 T cells in the lungs and relevant lymphoid tissues of Mtb infected mice. Aim 2 will address whether IFNg promotes contraction and/or turnover of CD4 T cells during Mtb infection. In Aim 3 we will address the direct effects of IFNg on CD4 T cells at the molecular level. PUBLIC HEALTH RELEVANCE: The emergence of antibiotic-resistant strains of Mtb, the lack of an effective vaccine and the growing population of immunosuppressed patients (largely due to HIV-infection) contribute to the tuberculosis crisis. It is well-established that IFNg is an essential cytokine that controls Mtb infection, and that CD4 T cells are critical for secreting IFNg. However, our current knowledge of how IFNg contributes to CD4 T cell function is inadequate. Thus, an understanding of the role of IFNg in shaping CD4 T cell differentiation and function is critical for a complete picture of how protective CD4 T cell immunity is established and maintained during Mtb infection.